The fatal flaw in all of these startups was they just aggregated content. None of them solved the real problem of distilling the overwhelming amount of content online into the few thing(s) I should read.
I'm also not sure how much people would be willing to pay even for really good content curation even if such a thing exited. And the reality is that curation is hard because my interests on a given day depend on so many factors from how busy I am to my mood to some event that piqued my interest.